Low phosphorus water treatment methods

ABSTRACT

A water or wastewater treatment system to reduce phosphorus or other pollutant concentrations in water or wastewater may include an optional primary treatment stage in fluid communication with a biological secondary treatment stage. The secondary treatment stage may, in turn, be fluidly connected with a tertiary treatment stage, which may include a chemical treatment portion for reducing phosphorus or other pollutants to desired levels. The precipitated phosphorus and other solids or sludge produced from the chemical treatment portion may be recycled upstream for reuse in the primary, secondary, and/or tertiary treatment stages. Such recycle may reduce the amount of added chemicals required in the tertiary treatment stage to phosphorus or other pollutants to desired amounts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(e) the benefit of U.S.provisional patent application No. 60/917,478, filed on May 11, 2007,and entitled “Low Phosphorous Water Treatment Methods and Systems”,which is hereby incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present invention generally relates to water treatment, and morespecifically to removing phosphorus, nitrogen, heavy metals and otherpollutants from water and wastewater.

INCORPORATION BY REFERENCE

U.S. Pat. No. 6,946,073, entitled “Method For Treating Wastewater In AMembrane Bioreactor To Produce A Low Phosphorus Effluent” and issued toDaigger et al. on Sep. 20, 2005, is hereby incorporated by referenceherein in its entirety.

BACKGROUND

Reducing total phosphorus levels in wastewater to phosphorusconcentration levels of about 0.5 mg/L or less when using a biologicalsecondary treatment stage often requires use of a tertiary treatmentstage after the biological treatment stage. Such tertiary stages may usea chemical treatment system to remove the phosphorus from thebiologically treated water. The chemical treatment system may includeadding metal salts, such as aluminum and iron salts, to precipitate orotherwise insolubilize the phosphorus, and then separating theprecipitated and other insolubilizied phosphorus from the chemicallytreated water or wastewater. Although such chemical treatment systemscan reduce the total phosphorus concentrations to levels of 0.5 mg/L orless, the operating and other costs for these systems, including thecost of the chemical, space for storing the chemical, the disposal costsfor the solids formed by the chemical treatment and so on, are oftenhigh, making these chemical treatment systems relatively expensive tooperate. Similar issues for attaining very low phosphorus levels inother types of water may arise when using a biological treatment as partof the water treatment process.

Accordingly, improved water and wastewater treatment systems are neededin the art.

SUMMARY OF THE INVENTION

One embodiment of the present invention may take the form of a methodfor treating wastewater. The method may include biologically treating awastewater including a pollutant with a pollutant concentration toproduce a biologically treated wastewater and a first sludge, separatingat least a portion of the biologically treated wastewater from the firstsludge, chemically treating the at least a portion of the separatedbiologically treated wastewater using at least a metal salt to reducethe pollutant concentration and to produce a second sludge comprising atleast a portion of the metal salt, and contacting the at least a portionof the separated biologically treated wastewater with at least a portionof the second sludge at least prior to or during the chemical treatment.

Another embodiment of the present invention may take the form of amethod for treating wastewater. The method may include biologicallytreating a wastewater including a pollutant with a pollutantconcentration to produce a biologically treated wastewater and a firstsludge, separating at least a portion of the biologically treatedwastewater from the first sludge, chemically treating the at least aportion of the separated biologically treated wastewater using at leasta metal salt to reduce the pollutant concentration and to produce asecond sludge comprising at least a portion of the metal salt, andcontacting the wastewater with at least a portion of the second sludgeat least prior to or during the biological treatment.

Yet another embodiment may take the form of a system for treatingwastewater including a wastewater supply, a biological treatment system,and a chemical treatment system. The biological treatment system may bedownstream of the wastewater supply and in fluid communication with thewastewater supply. The chemical treatment system may be downstream ofthe biological treatment system and in fluid communication with thebiological treatment system. The chemical treatment system may include asludge outlet operatively associated with the biological treatmentsystem.

Still yet another embodiment may take the form of a method for treatingwater. The method may include biologically treating a water including apollutant with a pollutant concentration to produce a biologicallytreated water, chemically treating at least a portion of thebiologically treated water using a treatment chemical to reduce thepollutant concentration and to produce a sludge comprising at least aportion of the treatment chemical, and contacting the at least a portionof the biologically treated water with at least a portion of the sludgeat least prior to or during the chemical treatment. In some embodiments,the water may include wastewater.

A further embodiment may take the form of a method for treating water.The method may include biologically treating a water including apollutant with a pollutant concentration to produce a biologicallytreated water, chemically treating at least a portion of thebiologically treated water using at least a treatment chemical to reducethe pollutant concentration and to produce a sludge comprising at leasta portion of the treatment chemical; and contacting the water with atleast a portion of the sludge at least prior to or during the biologicaltreatment.

Still yet a further embodiment may take the form of a system fortreating water or wastewater. The system may include a biologicaltreatment system and a chemical treatment system. The chemical treatmentsystem may be downstream of the biological treatment system and in fluidcommunication with the biological treatment system. The chemicaltreatment system may include a sludge outlet operatively associated withthe biological treatment system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first example of a wastewater treatmentsystem for removing phosphorus or other pollutants from wastewater.

FIG. 2 is a schematic view of a second example of a wastewater treatmentsystem for removing phosphorus or other pollutants from wastewater.

FIG. 3 is a schematic view of a third example of a wastewater treatmentsystem for removing phosphorus or other pollutants from wastewater.

FIG. 4 is a schematic view of an example of a water treatment system forremoving phosphorus or other pollutants from water.

DETAILED DESCRIPTION

Described herein are various embodiments of wastewater or watertreatment systems for reducing phosphorus or other pollutantconcentrations in wastewater or water, respectively. The wastewatersystems may take the form of an optional primary treatment stage influid communication with a secondary treatment stage. The secondarytreatment stage may, in turn, be fluidly connected with a tertiarytreatment stage, which may include a chemical treatment portion forremoving a desired amount of phosphorus or other pollutants from thewastewater. The precipitated phosphorus and other solids or sludgeproduced from the chemical treatment portion may be recycled upstreamfor reuse in the primary, secondary, and/or tertiary treatment stages.Such recycling may reduce the amount of added chemicals required in thetertiary treatment stage to reduce phosphorus or other pollutantconcentrations to desired levels.

The water systems may take the form of an initial treatment stage influid communication with a biological treatment stage. The biologicaltreatment stage may, in turn, be fluidly connected with a tertiarytreatment stage, which may include a chemical treatment portion forremoving a desired amount of phosphorus or other pollutants from thewater. Like the wastewater systems, the precipitated phosphorus andother solids or sludge produced from the chemical treatment portion maybe recycled upstream for reuse in the initial, biological, and/ortertiary treatment stages. Similar to the wastewater treatment systems,such recycling may reduce the amount of added chemicals required in thetertiary treatment stage to reduce phosphorus or other pollutantconcentrations to desired levels.

FIG. 1 depicts a schematic view of a first example of a wastewatertreatment system 100 for removing phosphorus or other pollutants fromwastewater. With reference to FIG. 1, the wastewater treatment system100 may include a primary treatment stage 105 for receiving pre-treatedor raw wastewater 110, which may also be referred to as influent, and asecondary treatment 115 stage fluidly connected to the first treatmentstage 105. The first treatment stage 105 may be fluidly connected to thesecond treatment stage 115 by any known method or system to fluidlyconnect water or wastewater treatment stages or zones, including, butnot limited to, pipes, channels, conduits, weirs and so on.

The secondary treatment stage 115 may, in turn, be fluidly connected toa tertiary treatment stage 120. Like the fluid connection between thefirst and secondary treatment stages 105, 115, the secondary treatmentstage 115 may be fluidly connected to the tertiary treatment stage 120by any known method or system to fluidly connect water or wastewatertreatment stages or zones. The tertiary treatment stage 120 maydischarge treated wastewater 125, which may also be called treatedeffluent, to the environment or to another water system. If desired, aportion of the treated wastewater 125 may be recycled upstream forfurther treatment or use in the system. Any treated wastewater 125 thatis recycled may be returned to the system at any desired location,including, but not limited to, one or more locations that are upstreamof the primary treatment stage 105, within the primary treatment stage105, between the primary and secondary treatment stages 105, 115, withinthe secondary treatment stage 115, between the secondary and tertiarytreatment stages 115, 120, or within the tertiary treatment stage 120.

The primary treatment stage 105 may involve separating solids from thewastewater influent 110. To separate solids from the wastewater influent110, the primary treatment stage 105 may include one or more settlingtanks, clarifiers or other conventional or known structures forseparating suspended solids from a raw or pre-treated wastewaterinfluent. However, in some systems the primary treatment stage 105 maybe omitted or bypassed, and the wastewater 110 may be sent directly tothe secondary treatment stage 115.

The secondary treatment stage 115 may include biological or other typesof wastewater treatment systems to remove BOD (biochemical oxygendemand) and/or suspended solids to predetermined levels or percentages(e.g., to below 30 mg/L for BOD and/or to reduce BOD by at least 85%).Other pollutants, such as nitrogen, phosphorus, heavy metals and so on,may also be removed from the wastewater in the secondary treatment stage115. The biological treatment systems may include trickling filters,activated sludge tanks or basins, oxidation or other treatment ponds,membrane bioreactors, or other systems for treating wastewater usingaerobic and/or anaerobic microorganisms, or other biological approaches.

The biological treatment systems may further include settling tanks,clarifiers, membrane filters or other structures for separating sludge,suspended solids, chemical precipitates, and other solids from thebiologically treated wastewater. Any portion of the separated secondarytreatment sludge or other solids may be recycled upstream for reuse inthe process. Pipes, channels, pumps and any other known solids, slurryor sludge conveyance devices, systems or methods may be used totransport the sludge upstream for reuse. Any such recycled sludge may bereturned to the wastewater treatment system 100 at one or more locationsupstream of, or within, the secondary treatment stage 115. Anynon-recycled portions may be removed from the wastewater treatmentsystem 100 for disposal, land use, or other purposes. Prior to disposingor using the non-recycled portions, the non-recycled portions may betreated using any known sludge or biosolids treatment system or method130, including, but not limited to, thermal treatment, composting, andso on.

The effluent exiting the secondary treatment stage 115, separated fromthe solids using a solids-liquid separator, may be considered abiologically treated wastewater or effluent 135. The biologicallytreated wastewater or effluent 135 may have BOD or other pollutantamounts or concentrations that are at least less than a predeterminedamount or concentration, respectively.

The biological treatment systems may be supplemented by adding chemicalsor other additives to remove pollutants, such as nitrogen, phosphorus,and heavy metals, by precipitation, absorption, or other physical and/orchemical methods. The chemicals or other additives may include, but arenot limited to, metal salts and ion exchange resins. The chemicals orother treatment additives may be added at any suitable point prior to orwithin the secondary treatment stage 115, including to the activatedsludge tanks or basins, the settling tanks or clarifiers, and so on.

The tertiary treatment stage 120 may include treatment systems forremoving residual suspended solids, nitrogen, phosphorus, and heavymetals contained in the biologically treated effluent. The tertiarytreatment stage 120 may include reaction basins or zones for mixingchemicals to promote reactions to absorb and/or precipitate pollutantsremaining in the wastewater after treatment in the primary and secondarytreatment stages 105, 115. The tertiary treatment stage 120 may alsoinclude filters of a wide variety of configurations (such as granularand cloth media filters), settling tanks, clarifiers, membrane filtersand other systems for separating solids from liquids.

During treatment of the wastewater in the primary and secondarytreatment stages 105, 115 (or the secondary treatment stage 115 if theprimary treatment stage 105 is omitted), a portion of the totalphosphorus in the wastewater may be removed through various physical,chemical, and biological mechanisms, including sedimentation ofparticulate phosphorus, chemical precipitation and settling, uptake forcell synthesis by microorganisms, and enhanced biological phosphorusremoval. Depending upon the configuration and treatment methods used inthe primary and secondary treatment stages 105, 115, the total averagephosphorus concentration in the biologically treated effluent 135exiting the secondary treatment stage 115 may approach levels of 500μg/L or less, with some systems at times reaching levels of 20 μg/L.

However, below approximately 100 μg/L or so, phosphorus concentrationsmay be too low to support the aerobic and anaerobic microorganisms, orother desired biological activity, in the secondary treatment system115. Thus, a tertiary treatment stage 120 may often be necessary toreduce the average total phosphorus concentration to levels ofapproximately 100 to 500 μg/L, and may especially be necessary to reducethe average total phosphorus concentration to levels below 100 μg/L toavoid undesirably suppressing biological activity in the secondarytreatment stage 115.

In the tertiary treatment stage 120, phosphorus removing chemicals, suchas aluminum sulfate (alum), ferric and ferrous chloride, or ferric andferrous sulfate, may be added to the biologically treated effluent 135to further remove phosphorus. The phosphorus removing chemicals mayremove phosphorus by direct precipitation and/or by formation of metalhydroxide floc with a capacity to absorb phosphate and other materials.The phosphorus removing chemicals may be rapidly mixed into thebiologically treated effluent 135 to condition the phosphorusprecipitate for solids-liquid separation. Approaches for separating theprecipitated phosphorus and other solids may include gravitysedimentation (which may be assisted by the addition of ballastingmaterial), granular media filtration, membrane filtration, and othersolids-liquid separation methods.

At least a portion of the phosphorus removing chemicals, or the othersolids or sludge (which may contain the phosphorus removing chemicals),separated from the chemically treated effluent in the tertiary treatmentstage 120 may be recycled into the wastewater at one or more locationsupstream of the primary treatment stage 105, in the primary treatmentstage 105, between the primary and secondary treatment stages 105, 115,in the secondary treatment stage 115, between the secondary and tertiarytreatment stages 115, 120, or within the tertiary treatment stage 120.Pipes, channels, pumps and any other known solids, slurry or sludgeconveyance devices, systems or methods may be used to transport thephosphorus removing chemicals, solids and/or sludge 140 upstream forreuse. Such recycling forms a counter current flow of phosphorusremoving chemicals (which may include phosphorus precipitated orabsorbed to at least a portion of the phosphorus removing chemicals) tothe wastewater flow in the water treatment system 100. This phosphorusremoving chemicals counter current flow may reduce the amount ofchemicals required to reduce phosphorus concentrations to a desiredlevel compared to similar processes that do not recycle the separatedsolids. The reduction in chemicals may be up to approximately 33 percentor more.

The phosphorus removing chemicals used in the tertiary treatment stage120 may be added in amounts up to or greater than the amounts requiredto reduce the average total phosphorus concentration in the effluent 125discharged from the tertiary stage treatment 120 to a predeterminedlevel. The predetermined level may range from 10 μg/L or less to 100μg/L or less. In some systems, the average total phosphorusconcentration in the wastewater may be approximately 100 μg/L or moreafter treatment in the secondary treatment stage 115 and may approach 10μg/L or less after treatment in the tertiary treatment stage 120. Inother systems, the average total phosphorus concentration in thewastewater may be approximately 20 μg/L or more after treatment in thesecondary treatment stage 115 and may approach 10 μg/L or less aftertreatment in the tertiary treatment stage 120. However, for at least thereasons discussed above, it is less likely systems employing biologicaltreatment in the secondary treatment stage 115 will produce wastewaterswith phosphorus concentration approaching 20 μg/L after treatment of thewastewater in the secondary treatment stage 115.

In operation, wastewater 110 may flow to the primary treatment stage 105for initial removal of suspended solids. From the primary treatmentstage 105, pre-treated wastewater 145 may then flow to the secondarytreatment stage 115 for biological treatment to reduce the amounts of atleast one of BOD or suspended solids. Other pollutants, such asnitrogen, phosphorus, heavy metals and so on, may also be reduced in thewastewater. The primary treatment stage 105, however, may be omitted.When omitted, the wastewater 110 may be initially and biologicallytreated in the secondary treatment stage 115. After biological treatmentin the secondary treatment stage 115, the biologically treated water maybe separated from the solids to form a biologically treated effluent anda biological or secondary treatment stage sludge. A portion of thesecondary treatment stage sludge 150 may be recycled upstream for reusein the secondary treatment stage 115. Any secondary treatment sludge notrecycled may be treated for disposal, or land or other use.

At least a portion of the biologically treated effluent 135 may thenflow to the tertiary treatment stage 120 for advanced or polishingtreatment to further reduce the amounts of suspended solids, nitrogen,phosphorus, heavy metals, or other pollutants contained in thebiologically treated effluent 135. Portions of the biologically treatedeffluent not treated in the tertiary treatment stage 120 may bedischarged to the environment, used in another water system, recycledback upstream for further treatment, or otherwise removed from or reusedin the system. The biologically treated effluent 135 received by thetertiary treatment stage 120 may be subjected to one or more chemicaltreatments to reduce phosphorus or other pollutant concentrations in thebiologically treated effluent 135. The chemicals may precipitate orotherwise insolubilize the phosphorus or other pollutants.

After chemical treatment in the tertiary treatment stage 120, thechemically treated biological effluent may be separated from thechemically formed solids or other sludge. The separated effluent may befurther treated in the tertiary treatment stage 120 to reduce otherpollutants not treated by the particular chemical treatment portion, ormay be discharged to the environment, used in another water system,recycled back upstream for further treatment, or otherwise removed fromor reused in the system. At least a portion of the chemically formedsolids or other tertiary stage sludge 140 may be recycled upstream forreuse in the primary, secondary, and/or tertiary treatment stages 105,115, 120. Any portions of the tertiary treatment solids or sludge 160not recycled may be treated for disposal, recovery of the addedchemicals, and so on. In some systems the tertiary treatment solids orsludge 160 may be combined with the secondary treatment sludge 155 fortreatment. After completion of the tertiary treatment stage 120, thetertiary treated effluent 125 may be discharged to the environment, usedin another water system, recycled back upstream for further treatment,or otherwise removed from or reused in the system.

FIG. 2 depicts a schematic view of a second example of a wastewatertreatment system 200 for removing phosphorus or other pollutants fromwastewater where like reference numbers may be used for elements thesame as or similar to elements of the first example of a wastewatertreatment system 100. Like the first example of a wastewater treatmentsystem 100, the second example of a wastewater treatment system 200 mayinclude a primary treatment stage 105 fluidly connected to a secondarytreatment stage 115, which may, in turn, be fluidly connected to atertiary treatment stage 120. Like the first example of a wastewatertreatment system 100, the primary, secondary and tertiary treatmentstages 105, 115, 120 for the second example of a wastewater treatmentsystem 200 may be fluidly joined by any known method or system tofluidly connect water or wastewater treatment stages or zones. Thesecond example of a wastewater treatment system 200 operates in the sameor a substantially similar manner as the first example of a wastewatertreatment system 100.

The primary treatment stage 105 may include one or more tanks or otherdevices for separating suspended solids from raw or pretreatedwastewater, and the secondary treatment stage 115 may be a conventionalactivated sludge system, or other biological water treatment system, forbiologically treating the wastewater. In some systems, the primarytreatment stage 105 may be omitted. The tertiary treatment stage 120 mayinclude two granular media filters 205, 210 in series for separatingprecipitated phosphorus and other solids from the chemically treatedeffluent 125.

Although two filters 205, 210 are depicted, more or less than twofilters may be used in the tertiary treatment stage 120. Further, thefilters may be arranged in series, in parallel, or in any combination ofseries and parallel alignments. Yet further, the filters may employcartridges or other filter materials in place of, or in combinationwith, the granular media. Still yet further, any other liquid-solidseparation device, system or approach may be used in place of, or incombination with, any or all of the filters.

Phosphorus removing chemicals, such as metal salts, may be added to thebiologically treated effluent 135 at one or more locations between thesecondary treatment stage 115 and the first filter 205. Phosphorusremoving chemicals may also be added to the wastewater at one or morelocations between the first and second filters 205, 210. At least aportion of the phosphorus removing chemicals or tertiary treatmentsludge 140 separated by the second filter 210 may be recycled to one ormore locations between the secondary treatment stage 115 and the firstfilter 205. Similarly, at least a portion of the phosphorus removingchemicals or tertiary treatment sludge 140 separated by the first filter205 may be recycled to one or more locations upstream of the primarytreatment stage 105 and/or between the primary and secondary treatmentstages 105, 115.

Although not shown, at least a portion of the phosphorus removingchemicals or tertiary treatment sludge 140 from the second filter 210may be recycled to one or more locations upstream of the primarytreatment stage 105, between the primary and secondary treatment stages105, 115, between the first and second filters 205, 210, or within theprimary or secondary treatment systems 105, 115. Also, at least aportion of the phosphorus removing chemicals or tertiary treatmentsludge 140 from the first filter 205 may be recycled to one or morelocations between the secondary treatment stage 115 and the first filter205 or within the primary or secondary treatment systems 105, 115. Likethe first example of a wastewater system 100, any portions of thetertiary treatment solids or sludge 160 not recycled for the secondexample of a wastewater treatment system 200 may be treated fordisposal, recovery of the added chemicals, and so on.

FIG. 3 depicts a schematic view of a third example of a wastewatertreatment system 300 for removing phosphorus or other pollutants fromwastewater where like reference numbers may be used for elements thesame as, or similar to elements, of the first and second examples ofwastewater treatment systems 100, 200. Like the first and secondexamples of wastewater treatments systems 100, 200, the third example ofa wastewater treatment system 300 may include a primary treatment 105stage fluidly connected to secondary treatment stage 115, which may, inturn, be fluidly connected to a tertiary treatment stage 120. Theprimary treatment stage 105 may include a tank or other structure forseparating suspended solids from raw or pretreated wastewater 110, andthe secondary treatment stage 115 may include a membrane bioreactor 305.The tertiary treatment stage 120 may also include at least a portion ofthe membrane bioreactor 305. In some systems, the primary treatmentstage 105 may be omitted.

The membrane bioreactor 305 may be any membrane bioreactor described inU.S. Pat. No. 6,946,073 (the '073 patent), or any known or commerciallyavailable membrane bioreactor system. The membrane bioreactors describedin the '073 patent use a combination of biological and chemicalphosphorus removal mechanisms to reduce phosphorus levels in wastewater.Pilot testing of these systems demonstrate that they can reliably reducetotal phosphorus concentrations below 100 μg/L and can approach levelsof approximately 20 μg/L. To reach average phosphorus concentrationlevels of 10 μg/L or less, however, an additional chemical addition andfiltering step in the tertiary treatment stage 120 may be necessary.

Like the first and second examples of wastewater treatment systems 100,200, the tertiary treatment stage 120 of the third example of awastewater treatment system 300 may include addition of phosphorusremoving chemicals at any location within the tertiary treatment stage120 to reduce phosphorus concentrations to a desired level. The addedchemicals may be any suitable chemical for reducing or removingphosphorus, including any chemicals described in the previous examplesof wastewater treatment systems 100, 200. A filter 310 may be used toseparate the precipitated phosphorus and other solids from thechemically treated effluent. The filter 310 may be any suitablesolids-liquid separator, including any described for the previousexamples of wastewater treatment systems 100, 200.

At least a portion of the separated phosphorus removing chemicals ortertiary treatment sludge 140 may be recycled to one or more locationsupstream of the primary treatment stage 105, between the primary andsecond treatment stages 105, 115, or directly to the membrane bioreactor305. Although not shown, at least a portion of the phosphorus removingchemicals or tertiary treatment sludge 140 may also be recycled to oneor more locations upstream within the primary or secondary treatmentsystems 105, 115, between the secondary and tertiary treatment systems115, 120, or within the tertiary treatment system 120. Like the firsttwo examples of wastewater systems 100, 200, any portions of thetertiary treatment solids or sludge 160 not recycled may be treated fordisposal, recovery of the added chemicals, and so on.

Chemicals may optionally be added to the system 300 at one or morelocations upstream of the primary treatment stage 105, between theprimary and second treatment stages 105, 115, and/or within thesecondary treatment stage 115 (including within the membrane bioreactor305). The chemicals may include phosphorus removing chemicals, such asany of the metal salts described above.

FIG. 4 depicts a schematic view of an example of a water treatmentsystem 400 for removing phosphorus or other pollutants from water, suchas arsenic or heavy metals. The water treatment system 400 may includean initial treatment stage 405 in fluid communication with a biologicaltreatment stage 410, and a polishing or tertiary treatment stage 415 influid communication with the biological treatment stage 410. The initialtreatment stage 405 may include water treatment devices and approachessimilar to the primary treatment stages 105 for the examples ofwastewater systems 100, 200, 300, described above and also a widevariety of devices and processes typically included in water treatmentapplications. The biological treatment stage 410 may include biologicalactivated carbon filtration or any other biological treatment approach.

Like the wastewater treatment systems 100, 200, 300, previouslydescribed, the tertiary treatment stage 415 for the water treatmentsystem 400 may include systems for removing residual suspended solids,nitrogen, phosphorus, and heavy metals contained in the biologicallytreated effluent 420. For example, the tertiary treatment stage 415,like the tertiary treatment stages 120 in the wastewater treatmentssystems 100, 200, 300, may include reaction basins or zones for mixingchemicals to promote reactions to absorb and/or precipitate pollutantsremaining in the water after initial and biological treatment. Thetertiary treatment stage 415 may also include filters of a wide varietyof configurations (such as granular and cloth media filters), settlingtanks, clarifiers, membrane filters and other systems for separatingsolids from liquids.

The tertiary treatment stage 415 depicted in FIG. 4 shows two filters425, 430 in series with chemical addition prior to each filter like thesecond example of a wastewater treatment system 200. Like the previousexamples of wastewater treatment systems 100, 200, 300, however, thetertiary treatment stage 415 of the water treatment system 400 mayutilize any tertiary treatment systems or approaches, or combination ofsystems and approaches. Thus, the tertiary treatment stage 415illustrated in the FIG. 4 is merely illustrative of one possibletertiary treatment system and is not intended to imply a particularsystem or to otherwise be limiting.

Like the second wastewater treatment system 200, treatment chemicals,such as metal salts, may be added to the biologically treated effluent420 at one or more locations between the biological treatment stage 410and the first filter 425. Treatment chemicals may also be added to thewastewater at one or more locations between the first and second filters425, 430. At least a portion of the treatment chemicals or tertiarytreatment sludge 140 separated by the second filter 430 may be recycledto one or more or locations between the biological treatment stage 410and the first filter 425. Similarly, at least a portion of the treatmentchemicals or tertiary treatment sludge 140 separated by the first filter425 may be recycled to one or more locations upstream of the initialtreatment stage 405 and/or between the initial and the biologicaltreatment stages 405, 410.

Also like the second wastewater treatment system 200, at least a portionof the treatment chemicals or tertiary treatment sludge 140 from thesecond filter 430 may be recycled to one or more locations upstream ofthe initial treatment stage 405, between the initial and biologicaltreatment stages 405, 410, between the first and second filters 425,430, or within the initial or biological treatment systems 425, 430.Also, at least a portion of the treatment chemicals or tertiarytreatment sludge 140 from the first filter 425 may be recycled to one ormore locations between the biological treatment stage 410 and the firstfilter 425 or within the initial or biological treatment systems 405,410. Like the examples of wastewater treatment systems described above,any portions of the tertiary treatment solids or sludge 160 not recycledmay be treated for disposal, recovery of the added chemicals, and so on.

Although the systems above are described with respect to using chemicalsto reduce phosphorus concentrations, any water or wastewater treatmentsystem which follows a biological treatment with a tertiary or polishingtreatment that utilizes metal salts or other similar chemicals to reduceother pollutants may include recycling of the separated chemicalsupstream in a manner similar to those described for the phosphorusreducing chemicals. Such pollutants may include, but are not limited to,heavy metals, such as arsenic and the like.

In methodologies directly or indirectly set forth herein, various stepsand operations are described in one possible order of operation, butthose skilled in the art will recognize that steps and operations may berearranged, replaced, or eliminated or have other steps inserted withoutnecessarily departing from the spirit and scope of the presentinvention. It is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative only and not limiting. Changes in detail or structuremay be made without departing from the spirit of the invention asdefined in the appended claims.

1. A method for treating wastewater comprising: biologically treating awastewater including a pollutant with a pollutant concentration toproduce a biologically treated wastewater and a first sludge; separatingat least a portion of the biologically treated wastewater from the firstsludge; chemically treating the at least a portion of the separatedbiologically treated wastewater using at least a metal salt to reducethe pollutant concentration and to produce a second sludge comprising atleast a portion of the metal salt; and contacting the at least a portionof the separated biologically treated wastewater with at least a portionof the second sludge at least prior to or during the chemical treatment.2. The method of claim 1, wherein the pollutant includes phosphorus. 3.The method of claim 2, wherein the pollutant concentration is greaterthan approximately 20 μg/L prior to chemically treating the at least aportion of the separated biologically treated wastewater and is lessthan approximately 10 μg/L after chemically treating the at least aportion of the separated biologically treated wastewater.
 4. The methodof claim 1, further comprising separating the chemically treatedwastewater from the second sludge prior to contacting the at least aportion of the separated biologically treated wastewater with the secondsludge.
 5. The method of claim 1, further comprising: subjecting atleast a portion of the chemically treated wastewater to a secondchemical treatment using a second metal salt to further reduce thepollutant concentration and to produce a third sludge comprising atleast a portion of the second metal salt; and contacting the at least aportion of the separated biologically treated wastewater with at least aportion of the third sludge prior to the first chemical treatment. 6.The method of claim 5, further comprising separating the secondchemically treated wastewater from the third sludge prior to contactingthe at least a portion of the third sludge with the at least a portionof the separated biologically treated wastewater.
 7. The method of claim1, wherein biologically treating the wastewater includes treating thewastewater using a membrane bioreactor.
 8. The method of claim 1,further comprising contacting the wastewater with at least a portion ofthe second sludge prior to the biological treatment.
 9. A method fortreating wastewater comprising: biologically treating a wastewaterincluding a pollutant with a pollutant concentration to produce abiologically treated wastewater and a first sludge; separating at leasta portion of the biologically treated wastewater from the first sludge;chemically treating at least a portion of the separated biologicallytreated wastewater using at least a metal salt to reduce the pollutantconcentration and to produce a second sludge comprising at least aportion of the metal salt; and contacting the wastewater with at least aportion of the second sludge at least prior to or during the biologicaltreatment.
 10. A method for treating water comprising: biologicallytreating a water including a pollutant with a pollutant concentration toproduce a biologically treated water; chemically treating at least aportion of the biologically treated water using a treatment chemical toreduce the pollutant concentration and to produce a sludge comprising atleast a portion of the treatment chemical; and contacting the at least aportion of the biologically treated water with at least a portion of thesludge at least prior to or during the chemical treatment.
 11. Themethod of claim 10, wherein the water comprises wastewater.
 12. Themethod of claim 11, wherein the treatment chemical comprises a metalsalt.
 13. The method of claim 12, wherein the pollutant includesphosphorus.
 14. The method of claim 13, wherein the pollutantconcentration is greater than approximately 20 μg/L prior to chemicallytreating the at least a portion of the separated biologically treatedwater and is less than approximately 10 μg/L after chemically treatingthe at least a portion of the separated biologically treated water. 15.A method for treating water comprising: biologically treating a waterincluding a pollutant with a pollutant concentration to produce abiologically treated water; chemically treating at least a portion ofthe biologically treated water using at least a treatment chemical toreduce the pollutant concentration and to produce a sludge comprising atleast a portion of the treatment chemical; and contacting the water withat least a portion of the sludge at least prior to or during thebiological treatment.
 16. The method of claim 15, wherein the treatmentchemical comprises a metal salt.
 17. The method of claim 15, wherein thepollutant includes phosphorus.
 18. The method of claim 15, wherein thewater comprises wastewater.